Recently, the refrigerant R744, i.e., carbon dioxide, has been employed in motor vehicle air conditioners instead of the conventional refrigerants R134a and R12. The use of carbon dioxide allows discharge of the refrigerant from the air conditioning system and venting to the atmosphere, unlike the conventional refrigerants. To protect system components from possible damage, it is usually recommended that the discharge rate be limited to some value. This value is typically expressed as a maximum pressure drop per minute such as 1 bar per minute.
Typically, discharge of carbon dioxide from an air conditioning system, such as used in an automobile, is accomplished by employing a fixed orifice that limits the flow rate of the discharging carbon dioxide. A block diagram of a conventional arrangement is depicted in FIG. 1, in which an air conditioning system 10 containing carbon dioxide to be discharged is connected to the fixed orifice 12. The carbon dioxide is discharged through the output 14 of the fixed orifice 12.
FIG. 2 shows an exemplary plot of the discharge rate (the Y-axis) vs. time (on the X-axis). It can be seen that the discharge rate decreases with time. The fixed orifice 12 prevents discharge rate from exceeding a safe maximum rate for the highest output pressure expected from the air conditioning system 10. However, one of the concerns for such a discharge arrangement is the relatively long time required to entirely discharge the air conditioning system 10. The long discharge time is a result of the refrigerant flow rate dropping due to reduced pressure differential across the fixed orifice 12. In other words, as carbon dioxide is discharged from the air conditioning system 10, output pressure from the air conditioning system 10 drops, causing input pressure at the fixed orifice 12 to drop and the pressure differential between the input of the fixed orifice 12 and the output of the fixed orifice 12 to be reduced. This reduces the discharge rate and increases the total discharge time, as depicted in FIG. 2. Even if the orifice were made adjustable to allow the discharge rate to be adjusted during a vehicle discharge, it would require an operator to be present to monitor and adjust the discharge rate.
There is a need for reducing the total discharge time for discharging carbon dioxide or other refrigerants from systems in a manner that still maintains integrity of the system and does not need to be tended to.